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Patterns in macroevolution (4 of 5) Lineage splitting

Image caption:Patterns of lineage-splitting (or speciation) can be identified by constructing and examining a phylogeny. The phylogeny might reveal that a particular lineage has undergone unusually frequent lineage-splitting, generating a "bushy" tuft of branches on the tree (Clade A, below). It might reveal that a lineage has an unusually low rate of lineage-splitting, represented by a long branch with very few twigs coming off (Clade B, below). Or it might reveal that several lineages experienced a burst of lineage-splitting at the same time (Clade C, below).

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Patterns in macroevolution (2 of 5) Character changeLineages can change quickly or slowly. Character change can happen in a single direction, such as evolving additional segments, or it can reverse itself by gaining and then losing segments. Changes can occur within a single lineage or across several lineages. Lineage A changes rapidly but in no particular direction. Lineage B shows slower, directional change.

Patterns in macroevolution (3 of 5) Trilobite exampleTrilobites, animals in the same clade as modern insects and crustaceans, lived over 300 million years ago. Their fossil record clearly suggests that several lineages underwent similar increases in segment number over the course of millions of years.

Patterns in macroevolution (5 of 5) Character changeExtinction can be a frequent or rare event within a lineage, or it can occur simultaneously across many lineages (mass extinction). Here, a mass extinction cuts short the lifetimes of many species, and only three survive.